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One of the staples of network defense is blocking traffic to and from a list of "known bad" sites on the Internet. However, few organizations are in a position to produce such a list themselves, so pragmatically this approach depends on the existence of third-party "threat intelligence" providers who specialize in distributing feeds of unwelcome IP addresses. However, the choice to use such a strategy, let alone which data feeds are trusted for this purpose, is rarely made public and thus little is understood about the deployment of these techniques in the wild. To explore this issue, we have designed and implemented a technique to infer proactive traffic blocking on a remote host and, through a series of measurements, to associate that blocking with the use of particular IP blocklists. In a pilot study of 220K US hosts, we find as many as one fourth of the hosts appear to blocklist based on some source of threat intelligence data, and about 2% use one of the 9 particular third-party blocklists that we evaluated.